Eight human volunteers were infected with S. mutans strains Ingbritt (serotype c) and OMZ 65 (serotype d/g) which were resistant to streptomycin; the level of implantation and duration of colonization were monitored. After both serotypes were shed from the mouth, the subjects were immunized against OMZ 65 by swallowing 100mg of formalin killed bacteria in capsules for three successive days and the subjects were reinfected with strains Ingbritt and OMZ 65. The level of implantation and duration of colonization were again monitored. After the bacteria were shed the immunization and implantation cycle was repeated except that capsules were ingested on seven successive days. The presence of antibody was accompanied by reduced implantation and colonization of OMZ 65.